Mitigation of Eutrophication in a Shallow Lake: The Influences of Submerged Macrophytes on Phosphorus and Bacterial Community Structure in Sediments

Effects of ferrous addition to Vallisneria natans: An attempt to apply ferrous to submerged macrophyte restoration

Restoration of submerged macrophytes is an efficient way for endogenous nutrient control and aquatic ecological restoration, but slow growth and limited reproduction of submerged macrophytes still exist. In this research, the effect of ferrous on the seed germination and growth of Vallisneria natans (V. natans) were studied through aquatic simulation experiments and its influence on the rhizosphere microbial community was also explored. The seed germination, growth, and physiological and ecological parameters of V. natans were significantly affected by the ferrous treatments. Ferrous concentration above 5.0 mg/L showed significant inhibition of seed germination of V. natans and the best concentration for germination was 0.5 mg/L. During the growth of V. natans, after ferrous was added, a brief period of stress occurred, which then promoted the growth lasting for about 19 days under one addition. The diversity and richness of the rhizospheric microbial were increased after the ferrous addition. However, the function of the rhizospheric microbial community showed no significant difference between different concentrations of ferrous adding in the overlying water. Ferrous addition affected the growth condition of plants (content of CAT, Chl a, Chl b, etc.), thus indirectly affecting the rhizospheric microbial community of V. natans. These impacts on V. natans and rhizosphere microorganisms could generalize to other submerged macrophytes in freshwater ecosystems, particularly which have similar habits. These findings would contribute to the ecological evaluation of ferrous addition or iron-containing water, and provide a reference for submerged macrophytes restoration and ecological restoration in freshwater ecosystems.

Interactions of Vallisneria natans and Iron-Oxidizing Bacteria Enhance Iron-Bound Phosphorus Formation in Eutrophic Lake Sediments

Submerged macrophyte restoration and in situ phosphorus (P) passivation are effective methods for the control of internal P loading from sediments. This study explored the synergistic effects of Vallisneria natans and iron (Fe)-oxidizing bacteria (IOB) on internal P loading from eutrophic freshwater lake sediments by taking into account Fe-bound P (FeP) formation and associated bacterial community structures. Sediment samples were prepared in glass tanks under four treatments, namely no V. natans planting or IOB inoculation (control), planting V. natans without IOB inoculation (Va), planting V. natans with IOB inoculation (Va-IOB), and planting V. natans with autoclaved IOB inoculation (Va-IOB[A]). Compared with the control, all three treatments with V. natans (Va, Va-IOB, and Va-IOB[A]) had significantly decreased organic matter contents and increased redox potential in sediments (p < 0.05), at the rapid growth and mature stages of V. natans. Planting V. natans with and without IOB inoculation also decreased the total P (TP) and Fe–P concentrations in sediments. Conversely, Fe³⁺ concentrations, Fe³⁺/Fe²⁺ ratios, and the proportions of Fe–P in TP all increased in sediments planted with V. natans, especially under the Va-IOB treatment (p < 0.05). Furthermore, bacterial community diversity increased in sediments due to the presence of V. natans. The relative abundances of IOB (including Acidovorax and Chlorobium) increased from the transplanting to the rapid growth stage of V. natans and then decreased afterwards. In the later stages, the relative abundances of IOB and their ratios to Fe-reducing bacteria were the highest under the Va-IOB treatment. Accordingly, synergistic interactions between V. natans and IOB could enhance Fe–P formation and reduce TP concentrations in eutrophic lake sediments by altering sediment physicochemical properties and Fe oxidation-related bacterial community structures.

Evaluation of Soil and Irrigation Water Quality in Caohai Lakeside Zone

Due to the rapid population growth and over-application of fertilizers in the Caohai surrounding farmlands, controlling the non-point source pollution in the Caohai Lakeside Zone is significant for the local ecology balance and human health safety. A total of 54 soil and 24 irrigation water samples were collected in the Caohai Lakeside Zone to evaluate the current status of soil fertility and heavy metal pollution by measuring soil and water heavy metal content and soil physicochemical properties such as soil total nitrogen, total phosphorus, organic matter, and soil pH. These results showed that the total amounts of organic matter, nitrogen, phosphorus, potassium, and effective nutrient content in the Caohai Lakeside Zone were all at a rich level according to Chinese soil nutrient grade standard; the content of lead (Pb), chromium (Cr), cadmium (Cd), mercury (Hg), and arsenic (As) in the soil all exceeds the safety standard of Chinese classification of soil environmental quality assessment. In addition, the over-standard rates of Cd and Cr were 24.1% and 14.8%, respectively. On the whole, the comprehensive quality index (CQI) of the soil in the Caohai Lakeside Zone is 6.48, which is attributed to the heavy Cd pollution and the good soil fertility. The heavy metal content of the irrigation water met the requirements of irrigation water quality. It is feasible to use the irrigation water of Caohai Lake in the actual agricultural production process. Therefore, in terms of the comprehensive management and pollution control of Caohai, special attention should be paid to the control of Cd, Pb, As, and Hg pollution sources.